1. Field of the Invention
The present invention relates to pelletized pre-dope granules of cellulose and a tertiary amine oxide wherein the tertiary amine oxide is homogeneously dispersed in the cellulose to penetrate therethrough, and a process for making them. Also, the present invention relates to a process for making a dope solution of cellulose adapted to be spun into cellulosic articles such as fibers and films.
2. Description of the Prior Art
A process for making cellulosic fibers using a tertiary amine oxide has greater advantages than the conventional processes using a viscose solution because the tertiary amine oxide is a non-polluting solvent. The process using a tertiary amine oxide can provide a clean environment for workers and substantially reduce the amount of waste water and atmospheric pollution which may occur in the course of producing fibers. Further, the viscose process is very complicated and requires high energy consumption, whereas the tertiary amine oxide is completely recovered. Thus, the latter process is more economical than the former one in view of the production cost. Moreover, the fibers obtained by the latter process are superior to those obtained from viscose rayon in view of their physical properties such as tensile strength. Thus, the latter fibers can be used as synthetic fibers with various advantages.
A process for making shaped cellulosic fibers from the cellulose solution in a tertiary amine oxide solvent was first proposed by Graenacher et al. in U.S. Pat. No. 2,179,181 issued in 1939. In accordance with the proposed process, from 7 to 10% by weight of cellulose is dissolved in the tertiary amine oxide solvent.
Since then, a number of methods have been proposed to develop a more effective and economical process for making cellulosic fibers. Most of them were focused on the production of a solution with a high solid content and the elimination of the problems of low workability due mainly to a high concentration of the solution. For example, a process using a N-methylmorpholine-N-oxide solvent as disclosed in U.S. Pat. No. 3,447,939 to Johnson, a process using a dimethyl sulfoxide, N-methyl-pyrrolidone, or sulfolane diluent for the solution in U.S. Pat. No. 3,508,941 to Johnson, and processes using amines as a co-solvent in U.S. Pat. Nos. 4,290,815 to Henry and 4,324,539 to Vargar, have been proposed.
Processes for producing a solution having a high solid content of cellulose in an amine oxide containing an appropriate amount of water are disclosed in U.S. Pat. Nos. 4,142,913 to McCorsley et al.; 4,144,080 to McCorsley; 4,196,282 to Franks et al.; and 4,246,221 to McCorsley. However, since the solution according to the above patents has too high viscosity to be wet-spun, these patents employ an air-gap spinning technique for increasing the production yields.
In accordance with the processes disclosed in these patents, cellulose is added to a solution of a tertiary amine oxide containing 50% or more excess of water to form a slurry by swelling, and then an excess of water contained in the slurry is distilled off under reduced pressure to form a cellulose solution for spinning. The resulting solution is supplied to an extruder equipped with a spinnerette. However, these processes necessarily require a long time for the distillation under reduced pressure. This causes thermal decomposition of cellulose and consequently discoloration of the spinning solution with reduction of molecular weight. Moreover, the resulting solution has a disadvantage that it causes deterioration of some physical properties after spinning.
U.S. Pat. Nos. 4,142,913 to McCorsley et al., and 4,144,080 to McCorsley disclose an azeotropic distillation process in which a non-solvent such as, for example, alcohols, dimethylformamide, dimethylsulfoxide, toluene, and xylene is added in order to more easily remove the excess of water. However, since a large amount of the non-solvent materials contained in the removed water must be recovered for reuse, these processes have a drawback that an additional, complicated step is required to separate the non-solvent materials from water. In addition, in the course of concentrating the cellulose solution, the concentration temperature and time will be raised and extended with the increment of the cellulose concentration, and more energy will be required to stir the slurry which will have higher viscosity. Thus, it is difficult to carry out a continuous process and results in low productivity.
U.S. Pat. No. 4,211,574 to McCorsley et al. discloses a process for making a cellulose solution wherein a pulp sheet of cellulose is soaked in a liquid tertiary amine oxide having a water content of 5 to 15% cellulose at 89.degree. to 95.degree. C. to be swollen, and then the mixture is stirred and heated without concentration. This process has the advantage that a cellulose solution having a high solid content can be produced within a short period of time without concentration. However, since the liquid phase of amine oxide may swell the cellulose concurrently with the dissolution of the cellulose, it is may be likely to form a coating on the surface of pulp which is not yet dissolved. This makes a deep penetration of the amine oxide into the pulp impossible. Thus, this process has the drawback that, in spite of strong stirring or heating of the solution, a homogeneous solution cannot be formed due to the presence of undissolved cellulose.
European Patent No. 356,419 to Stefan et al. suggests a continuous process for making a large amount of cellulose solution within a short period of time by means of the same concentration steps as those of the above processes. According to this process, the cellulose slurry having a weight average polymerization degree of 1,400, which is obtained by swelling the cellulose solution in a tertiary anine oxide containing 40% by weight of water, is transferred to an extruder fitted with a screw having a fan type screw path, and distilled under reduced pressure with the retention time of 3 minutes in the extruder to form a cellulose solution at the rate of 72 kg per hour. In this process, the fan type screw is used for transferring the slurry concurrently with its concentration. Thus, this process has the problem of high production cost due to the energy for operating the screw.
Various processes for producing a cellulose solution in a tertiary amine oxide suggested in all the prior art patents as mentioned above may be categorized into two groups as follows:
Group I: Processes as disclosed in U.S. Pat. Nos. 4,142,913, 4,144,080, 4,196,282, 4,246,221, 4,290,815, and European Patent No. 356,419. ##STR1##
The cellulose solution obtained from the above prior art processes is highly viscous and is in a phase where the cellulose has been dissolved completely. Solidification of the solution takes several hours to several days. Thus, it is very difficult to convert the cellulose solution to chips. Moreover, even if the solution is converted into chips, decomposition of the cellulose and discoloration of the solution may occur during the additional dissolution steps which are carried out in an extruder to spin the solution. Thus, the solution must be transferred directly into an extruder in a liquid phase without any chipping steps. Otherwise it takes a long period of time to make chips.
As mentioned-above, the inventors of the present invention have extensively investigated the foregoing conventional processes for producing a cellulose solution in order to provide an practical process capable of forming a cellulosic article. As a result, the inventors have discovered that the above problems can be eliminated by milling and blending cellulose with a solid tertiary amine oxide (powder) having a specific content of water to provide pelletized pre-dope granules, wherein the tertiary amine oxide is homogeneously dispersed throughout the cellulose, followed by heating the pre-dope granules in the barrel of an extruder to dissolve the cellulose, extruding the resulting dope solution to provide an extrudate adapted to be spun, and spinning the extrudate through a spinnerette or die. The present invention has been completed based on this discovery.